Previous research efforts have led to the synthesis of many conjugated polymers, particularly derivatives of poly(phenylene vinylene) (PPV), soluble in organic solvents and easily processed into films with great promise as organic electronic materials. π-Conjugated oligomers, first investigated as model compounds for conjugated polymers, are now widely studied for use in optoelectronic devices because their well-defined chemical structure facilitates tuning of electronic properties. Use of well-defined oligomers reduces defect density while enabling more control over the effective conjugation length. In particular, oligo(p-phenylene vinylene)s (OPVs) are being investigated for use in solar cells and light emitting diodes (LEDs) due to their stability, high luminescent efficiency and ease of synthesis. OPVs with solubilizing substituents combine the low-cost, solution-based processing of conjugated polymers with the improved structural control inherent to oligomers.
Controlling the nanoscale structure of rod-like conjugated polymers has proven difficult. Supramolecular order plays a critical role in device performance, as both charge mobility and luminescent efficiency are influenced by molecular aggregation and structural defects. One approach to control nanostructure has been to design polymers that could exhibit thermotropic or lyotropic liquid crystalline (LC) behavior. Among these are the “hairy-rod” type liquid crystalline polymers (LCPs), in which flexible alkyl side chains added for solubility induce nanophase separation of the rigid, conjugated polymer backbone. A more direct approach is to incorporate calamitic mesogens as side chains on a conjugated polymer backbone. Both methods can result in layered, smectic ordering of the polymer, but only micron-sized domains are generally obtained due to the viscosity and rigidity of the polymer's extended backbone conjugation, which inhibits formation of the large monodomains needed in device applications.
Use of conjugated, liquid crystalline oligomers can improve ordering, and several LC oligomers based on substituted OPV rods have been reported. Many are analogs of conjugated LCPs, with flexible alkyl chains grafted laterally onto the molecule. However, such substituent configurations transverse to a longitudinal molecular axis introduce bulky, sterically hindering groups that disrupt co-planarity within the π-conjugated system.